1. Field of the Invention
The invention relates to a braking circuit for an electric motor, especially for driving electric hand-tools such as drills, angle grinders, hedge trimmers and the like, having a braking switch, and to an electrical switch, which is suitable for use in the braking circuit, having at least two contact systems which are constructed as changeover switches.
2. Description of the Related Art
Electric motors have a relatively long running-on time after the power supply voltage has been switched off, as a result of their kinetic energy. Particularly when used in electric hand-tools, such as angle grinders, chainsaws, electric planes and the like, because of the possible risk of injury to the operator it is desirable to ensure rapid braking of the electric motor, and hence of the electric hand-tool, when the electric motor is switched off. The electric motor is normally braked by means of a braking circuit in which the armature and the field winding are switched into a braking circuit during switching off, that is to say when switching over from the motor mode to the braking mode. The kinetic energy is then converted into heat in the braking circuit,-.and emitted to the surrounding air.
DE-PS 35 46 719 discloses such a braking circuit for a universal electric motor, in the case of which a braking circuit is formed by the electric motor armature being short-circuited via the field in the switched-off state. For this purpose, a braking switch comprising two changeover switches S1, S2 is provided the connections for the switching contacts of which braking switch are connected to the two connections of the field winding. The connections of the stationary contacts of the two changeover switches S1, S2 are connected to one power supply terminal and to the two sides of the armature winding such that, when the electric motor is in the switched-off state, the polarity of the armature winding with respect to the field winding is the opposite of that when the electric motor is in the switched-on state.
In order to prevent the contacts of the braking switch from being destroyed as a result of high currents in the braking circuit when the electric motor is being switched off, or as a result of power supply short-circuits, it is proposed that the switching processes in the braking switch take place in a time-delayed manner. When switching over from the motor mode to the braking mode, the changeover switch S1, which is close to the power supply, is operated first and then the changeover switch S2 which is remote from the power supply is operated. When switching over from the braking mode to the motor mode, the changeover switch S2 which is remote from the power supply is operated first, and then the changeover switch S1 which is close to the power supply is operated.
Furthermore, electronic circuits are known which are arranged in the braking current path of the braking circuit and are used for controlling the braking of the electric motor. Such braking electronics are disclosed, for example, in DE-OS 3,539,841, DE-OS 3,636,555 and DE-OS 4,022,637. However, no more precise statements are made in these Laid-Open Specifications with respect to the braking switch, which once again comprises two changeover switches, and, in particular, to the sequence of the switching processes of the two changeover switches.
It has been found that, in the case, of a braking circuit having a braking switch of the specified type, failures can occur in the braking when switching over from the motor mode to the braking mode, especially when using braking electronics. In such a case, the electric motor then runs on without being braked. It can immediately be seen that, just for safety reasons, such a circuit, having a braking switch in which there is no absolute guarantee of a fault-free braking behavior, is unsuitable as a braking circuit.
It has furthermore been found that the braking switch described in DE-PS 3,546,719 has a tendency to have failures resulting from contact erosion after only a short operating period. Particularly when used on electric motors of relatively high power, for example over 1200 W, the contacts weld within a very short time and the braking switch becomes unusable. Even if switching operations from the braking mode directly back to the motor mode are made frequently, without waiting for the motor to stop, this problem occurs to a considerable extent.